The present invention relates to an apparatus for working a workpiece by relatively moving a rotating cutting tool and a table with the workpiece put thereon, and more particular, to an apparatus for working a workpiece wherein vibrations of the table are suppressed, which vibrations bring about a disadvantage such as degradation in machining accuracy and breakage of the cutting tool, in case of working a large-sized or lengthy workpiece.
In cutting work of a workpiece by the use of a rotating cutting tool, a part or parts, which constitute a machine tool, suffers self-excited vibrations in the case where the machine tool and the workpiece are small in dynamic stiffness (stiffness in motion), or in the case where a cutting force acting between the cutting tool and the workpiece is large. Self-excited vibrations as generated are transmitted to the cutting tool with the result that a worked surface of the workpiece becomes wavy and increases in surface roughness, thus making it impossible to obtain a desired machining accuracy. Further, when the self-excited vibrations increase, there is brought about a disadvantage that a cutting blade of the cutting tool itself is nicked, or increased in abrasion, and the cutting tool itself is eventually shortened in service life.
In the case where a workpiece is a large-sized or lengthy member, a structure of a machine tool used, for example, in cutting work of a railway rail over 20 meters, working of a high-accuracy guide rail (5 to 8 meters) of a conveyance stage used in manufacture of liquid crystal panels, or the like becomes necessarily large, so that it is desired to improve a constituent member or members in dynamic stiffness not to bring about self-excited vibrations at the time of working.
Generally, a large-sized, cutting machine tool described above frequently employs a ball screw system, which is combined with a rotating machine for traveling of a table with a workpiece put thereon. Of course, since the ball screw itself is increased in length, the ball screw itself is liable to suffer self-excited vibrations when the workpiece is worked, and the vibrations are transmitted to the table to make high-accuracy working of the workpiece difficult.
As one of countermeasures in such case, a part or parts, which constitute a machine tool, are increased in thickness to improve the machine tool in dynamic stiffness. Since such countermeasure makes the machine tool large-sized and heavy, however, not only a large amount of energy is required, for example, to move a heavy table but also much cost is needed in running of the apparatus.
As another countermeasure, a method described in JP-A-2007-203435 has been proposed. That is, in the case where a working head with a cutting tool mounted thereon is driven by a ball screw, an opposite side part of the ball screw with respect to the working head is formed with inverse threads and a weight is moved in an opposite direction with respect to the working head to cancel reaction generated when the working head is driven, thus suppressing vibrations acting on the ball screw.
Also, JP-A-10-202461 proposes a method of suppressing vibrations acting on a table by virtue of movements of a damper, which comprises a weight and a spring and is mounted on the table with a workpiece put thereon.
JP-A-02-243265 discloses a construction, in which a table travels on rails, that is, a stationary member (base) with the use of a ball screw and a damper provided in the table is used to absorb vibrations at that time.
According to the disclosure of the JP-A-2007-203435, variation in moving speed of the working head driven by the ball screw is cancelled but there is not provided a mechanism for suppressing vibrations suffered by a workpiece in working. Also, according to the disclosure of the JP-A-10-202461, a vibration absorbing body, that is, the damper, which comprises a weight and an elastic body (spring), mounted on the table is used to suppress vibrations accompanying movements of a ball screw. At this time, the heavier the weight, which constitutes the damper, the more effective the vibration damping effect, but it cannot but be said that since a member, which supports the table itself, is the ball screw itself for realization of feed of the table, it is difficult to suppress vibrations at the time of working large-sized or lengthy and heavy goods being an object of working in the invention.
On the other hand, JP-A-02-243265 discloses a structure for solving matters of bottlenecks in the JP-A-10-202461. That is, the table travels on the rail grooves provided on the stationary member so as to cope with working of heavy goods. At this time, the damper (comprising a weight and a spring) mounted in the table serves to suppress vibrations transmitted to the table. Since the weight is needed to move without suffering an unnecessary resistance in a vibratory direction, however, it must be arranged in no contact with a table inner wall and so there is involved a disadvantage that it is very difficult to make the weight heavy to improve a vibration damping effect.
As described above, in case of cutting work while using a ball screw system to move a table on which a large-sized workpiece or lengthy and heavy goods are put, there is involved a serious problem that the ball screw itself becomes necessarily lengthy and spatial displacement of the ball screw itself in operation is transmitted as vibrations to the table to bring about degradation in machining accuracy, or the like.